The present invention relates to a hybrid circuit adapted to couple a broadband modem to a telecommunication line, said hybrid circuit comprising a transformer bridge circuit including a bridge circuit coupled between a drive amplifier and a receive amplifier, and a transformer of which a secondary winding is electrically coupled to said telecommunication line,
said bridge circuit comprising first, second, third and fourth nodes interconnected via four legs,
a first leg comprising a first impedance matching network coupled across said second and third nodes,
a second leg comprising a first primary winding of said transformer coupled across said first and third nodes,
a third leg comprising a second impedance matching network coupled across said first and fourth nodes, and
a fourth leg comprising a second primary winding of said transformer coupled across said second and fourth nodes,
the secondary winding of said transformer being magnetically coupled to said first and second primary windings,
said drive amplifier having outputs connected to said first and second nodes and being adapted to provide, at said output, an output signal derived from a transmit signal received from said modem, and said receive amplifier having inputs coupled to said third and fourth nodes and being adapted to provide a receive output signal to be applied to said modem.
The hybrid circuit of the present invention is more particularly adapted for coupling a broadband modem to a telecommunication line that transports electrical signals in several frequency ranges that are not all used by the modem. The telecommunication line belongs for instance to a Plain Old Telephone System POTS or to an Integrated Services Digital Network ISDN, operating according to a Digital Subscriber Line protocol DSL, such as Asymmetric Digital Subscriber Line protocol ADSL, HDSL, HDSL2, RADSL, VDSL, etc . . . .
Due to the increasing popularity of Digital Subscriber Lines DSL, network access providers require Central Office CO equipments that allow a high line density on the printed circuit boards. Fulfilling this requirement implies a reduction of the power consumption per line and a reduction of the area occupied by the off-chip circuitry.
Two basic architectures of hybrid circuits were the most used in the past: the so-called “three-transformers hybrid” and the classic resistive hybrid.
The three-transformers hybrid has weak points related to both the above issues: part of the power delivered by the line drive amplifier is uselessly dissipated in a balanced impedance, and transformers are big components. The three transformers of this known architecture occupy thus a large area on the board.
The classic resistive hybrid has the drawback of providing a high attenuation from the line connection to the receive connection. This basic architecture has however been improved and, at the present time, a hybrid circuit providing better results is obtained by an architecture as described above.
Such an hybrid circuit is already known in the art, e.g. from the U.S. Pat. No. 6,163,579 (WO 9945655) entitled “Broadband modem transformer hybrid” by Harrington, et al., filed on Mar. 4, 1998. Therein, is disclosed a hybrid circuit adapted to be used in a broadband modem coupled to a telephone line. This hybrid architecture is compact and characterized by a low attenuation from the line connection to the receive connection. The area occupied on the board is limited by the surface of the hybrid. However, this known hybrid circuit is not adapted to perform impedance synthesis.
The synthesis of impedance is a method commonly used to decrease the power consumption and output voltage swing of a drive amplifier whose output impedance has to match with a given value.
It can be proved that if an impedance synthesis was used in the above known patent, the impedance of the matching network would be smaller than the impedance reflected in the primary windings of the transformer. This would result in an unacceptable low trans-hybrid loss. In order to recover an infinite theoretical trans-hybrid loss, each node connected to the receive amplifier needs to be moved from the extremity of the primary winding of the transformer towards an intermediate connection into this primary winding. However, such an intermediate connection in the primary winding of a transformer is inaccessible and, additionally, its position may also vary as a function of the load.